One-Dimensional Water-Hydroxyl Chain Complexes: Hydrogen-Atom Relay Reactions in Real Space - Visualization of Hydrogen-Bond Dynamics - page 114

10.2 Results and Discussions
10.2.1 Assembly of One-Dimensional Water-Hydroxyl
Chain Complexes on Cu(110)
Figure 10.2 show the assembling procedure of an H 2 O-(OH) 2 complex. First some
water molecules are dissociated into OH species and subsequently into atomic
oxygen (Fig. 10.2 a). In the next step a water molecule is manipulated along the
[110] direction to react with the oxygen atom, yielding an H-bonded hydroxyl
dimer, (OH) 2 (Fig. 10.2 b); the characterization and structure of (OH) 2 is described
in Chap. 8 ] . Finally, the other water molecule is brought to one end of the (OH) 2 ,
yielding an H 2 O-(OH) 2 chain-like complex (Figs. 10.2 d-f). An H 2 O-(OH) 3 chain
is assembled by moving an H 2 O-OH complex to an (OH) 2 (Fig. 10.3 ) [the
structure of H 2 O-OH is described in Chap. 9 ]. An H 2 O-(OH) 4 is assembled as
follows; First, two oxygen atoms separating by 2b 0 along [001] direction each
other are prepared (Fig. 10.4 a). Then two water molecules are sequentially reacted
with these oxygen atoms in the same way as Fig. 10.2 a-b. This reaction generates
an H-bonded (OH) 4 chain (Fig. 10.4 b-c), and finally the other water molecule is
attached to the end of the (OH) 4 , yielding an H 2 O-(OH) 4 complex (Fig. 10.4 d).
Every reaction described above takes place spontaneously when the reactants
come close to each other. It is noted the produced complexes are sufficiently stable
to be imaged with a low bias voltage.
In addition to 1-D chain configurations, two structural isomers of H 2 O-(OH) 2 can
be obtained depending on the initial position of a water molecule that reacts with an
(OH) 2 . A water molecule is attached to a hydroxyl that acts as the H-bond acceptor
and donor in Figs. 10.5 a-b and c-d, respectively. Because hydroxyl is expected to be
a strong H-bond acceptor, I propose the structures of the side-on complexes where the
water molecule donates its H bond to the hydroxyl group in (OH) 2 for both cases (the
tentative model is illustrated on the bottom of STM images of Fig. 10.5 b and d). A
high voltage pulse (*5 V) can transform these side-on complexes into the chain
configuration via several unidentified intermediate states.
10.2.2 Structure of an H 2 O-(OH) 2 Chain Complex on Cu(110)
The structure of H 2 O-(OH) 2 was determined by DFT calculations. 1 Figure 10.6 a
shows the optimized structure, where the water molecule and hydroxyl species are
1 The DFT calculations were carried out with a plane-wave basis set and ultrasoft pesudopo-
tential method [D. Vanderbilt, Phys. Rev. B 41, 7892 (1990).] as implemented in the STATE
code [Y. Morikawa et al. Phys. Rev. B 69, 041403(R) (2004).]. Perdew-Burke-Ernzerhof (PBE)
[J.P. Perdew et al. Phys. Rev. Lett. 77, 3865 (1996).] generalized gradient approximation (GGA)
was used for the exchange-correlation functional. Plane-wave cutoffs of 36 Ry and 400 Ry were